Sargassum spp. Seaweeds

Brown algae in the Genus Sargassum are synonymous with the Sargasso Sea, the clockwise gyre that surrounds Bermuda. Winds and currents in this slowly spinning body of water concentrate floating algae and drifting ocean creatures (and other marine debris like plastic) into large, shifting rafts. These Sargassum rafts in the heart of the Sargasso Sea form the basis of a unique high-seas community.

The two most abundant species in the waters around Bermuda are the Common Gulfweed (Sargassum natans) and Broad-toothed Gulfweed (Sargassum fluitans). Both of these species are holopelagic, meaning that they do not grow attached to a substrate like the seabed or shoreline. Instead they spend their entire life free-floating in the ocean in large mats. The Sargassum is kept afloat at the sea surface by small, round gas-filled bladders that grow along the stem of the plant. Pieces of the algae break off and begin to grow into new plants, while the old algae die and sink to the bottom of the ocean.

Common Gulfweed or Spiny Gulfweed (Sargassum natans)

Sargassum natans with attached hydroids

As the name suggests, Common Gulfweed is the most common Sargassum species found in the Sargasso Sea and washed up on Bermuda’s beaches. S. natans is a bushy seaweed with narrow leaf blades which are golden brown with toothed edges. The rubbery-textured leaves range from 2-6 mm (0.07-0.2”) wide and 2-10 cm (0.8-4”) long. The gas-filled floats are less than 6 mm (0.2”) in diameter and are held on short stalks along the stems among the leaves. The floats of S. natans have a single protruding spine 2-5mm (0.07-0.2”) long. S. natans does not have a single main stem; instead it grows in many directions forming clumps that can reach 60 cm (23.5”) long. It is these clumps that form together into much larger mats.

Broad-toothed Gulfweed (Sargassum fluitans)

S. fluitans very much resembles S. natans, as both are golden brown in colour, with toothed, rubbery leaves, small gas-filled floats and no central stem. The leaves of Broad-toothed Gulfweed, as the name suggests, are wider reaching up to 8 mm (0.3”) wide and 2-6 cm (0.8-2.4”) long. The gas bladders of S. fluitans are held on relatively long stalks along the centre of the plant. The surface of the floats is smooth.

Sargassum is at the mercy of the winds and currents, and sometimes arrives onshore in massive amounts that create a nuisance to swimmers, beach users and boats. The seaweed is however, an important component of Bermuda’s beach habitats. On the beach, Sargassum and its associated community of creatures, provide forage for birds and other creatures. The decaying seaweed also attracts beach-dwelling invertebrates that are preyed on as well.

Once washed ashore Sargassum dries in the sun, turning from golden brown, to a dark brown, to black and brittle when totally dry. Sargassum on the lower part of the beach is removed again by the waves and tides, but higher up towards the back of the beach the dried Sargassum becomes buried overtime. In this way Sargassum provides critical services for dune formation such as anchoring sand grains to stabilise dunes and providing fertiliser for sand dune plants.

In Bermuda Sargassum arrives on the beaches in varying amounts throughout the year. Traditional uses of Sargassum include gathering seaweed from beaches, washing the salt out and spreading it below banana trees as fertiliser.